1. Field of the Invention
The present invention relates to a tension apparatus for applying tension to an occupant restraining webbing in an emergency situation of a vehicle, thereby tightly restraining the occupant's body by the webbing.
2. Description of the Related Art
In a typical seatbelt system for a vehicle which is designed to protect an occupant in an emergency situation of the vehicle, the occupant is fastened by a webbing having one end thereof wound up into a retractor, and when an emergency situation of the vehicle occurs, the body of the occupant is restrained by the webbing.
The retractor applies a predetermined tension to the webbing. However, the force for winding up the webbing is controlled so as to be relatively weak or zero in a normal state in order to prevent the occupant from feeling pressure from the webbing. Therefore, there may be a slight gap between the webbing and the occupant's body. For this reason, it may be expected that, when an emergency situation of the vehicle occurs, the body of the occupant will move by inertia toward the front end of the vehicle by a distance corresponding to the gap during a period of time which starts when the webbing is stopped from being unwound and which ends when the occupant's body is reliably restrained by the webbing.
In order to prevent the occupant's body from moving in an emergency situation of the vehicle, one type of apparatus has heretofore been proposed wherein, when an emergency situation of the vehicle occurs, the biasing force derived from, for example, a spiral spring is transmitted to a webbing take-up shaft, thereby applying tension to the webbing (see, e.g., the specification of Japanese Utility Model Laid-Open No. 162856/1983).
This type of conventional apparatus is arranged such that, when the vehicle is in a normal state, the webbing is biased in a direction in which it is wound up by the biasing force from a relatively small resilient member, whereas, when an emergency situation of the vehicle occurs, the biasing force from a relatively large resilient member is transmitted to the webbing take-up shaft, thereby forcing the shaft to wind up the webbing. Accordingly, after the biasing force of the relatively large resilient member has been applied to the take-up shaft at the time of an emergency situation, this force continuously acts on the shaft, and this is inconvenient to reuse this apparatus.